1. Field of the Invention
The present invention relates to an optical head and a disk reproducing apparatus.
2. Description of the Related Art
Among disk reproducing apparatuses, some disk reproducing apparatuses are capable of performing recording and/or reproduction on plural types of magneto-optical recording media having different physical formats, i.e., MD (Mini Disc; registered trademark) and Hi-MD optical head provided in such a disk reproducing apparatus that (registered trademark). An performs at least reproduction on plural types of magneto-optical recording media includes a light source for emitting laser light, an objective lens for converging the laser light emitted by the light source onto an information recording surface of a magneto-optical recording medium, an optical system for separating laser light that is return light reflected on the information recording surface of the magneto-optical recording medium, and a signal conversion section for converting the laser light separated by the optical system into an electrical signal.
A magneto-optical recording medium such as an MD or Hi-MD has guide grooves simply referred to as grooves provided on an information recording surface thereof. When the magneto-optical recording medium is reproduced, a disk reproducing apparatus irradiates the grooves with laser light emitted by a light source and reads information recorded in the grooves from a reflection of the irradiating light. Recently, the track pitch of magneto-optical recording media is made smaller for higher density to allow information signals to be recorded on the magneto-optical recording media as much as possible.
MDs used in the related art have a track pitch of 1.6 μm, and Mi-MDs which have recently been developed to allow high density recording have a track pitch of 1.25 μm. EFM (Eight to Fourteen Modulation) data are recorded in the grooves of an MD, and data modulated on the basis of RLL(1-7) PP are recorded in the grooves of a Hi-MD, where RLL stands for “Run Length Limited”, and PP stands for “Parity preserve/Prohibit RMTR (Repeated Minimum Transition Run Length)”, and RLL(1-7) PP is a physical format for recording in a density higher than that on an MD. An optical head including a light source emitting laser light having a wavelength of 780 nm and an objective lens having a numerical aperture (NA) of 0.45 is used in compatibility with both of MDs and Hi-MDs which have different physical formats as thus described.
When such an optical head is used, the diameter of a spot of laser light emitted by the light source can become larger than the track pitch, and the spot diameter can extend beyond a groove. Such light extending beyond a groove is reflected by the surface of a land adjacent to the groove, and the reflection is included in light that is reflected by the groove so that both of the reflections on the land and groove are converted into electrical signals. Such a phenomenon is referred to as crosstalk. When the reflection on the groove includes another light, many errors can be generated in the electrical signal obtained by the conversion of such light, e.g., an information recording/reproduction signal (RF signal), whereby reproduction characteristics can be degraded.
Under the circumstance, proposals have been made on optical heads in which a phase compensation element is inserted into the light path of light reflected by a magneto-optical recording medium to reduce errors by suppressing crosstalk components from the lands and to thereby prevent degradation of reproduction characteristics (for example, see Japanese Unexamined Patent Publication JP-A 2003-296960 (pp. 14-15 and FIG. 16)).
FIG. 15 is a sectional view showing a schematic configuration of a related-art optical head 1 disclosed in JP-A 2003-296960. The optical head 1 which is a discrete optical system comprises a semiconductor laser element 2 for emitting laser light, a grating 3 for separating light emitted by the semiconductor laser element 2, a beam splitter 4 for transmitting or reflecting light incident thereon, a collimator lens 5 for converting light incident thereon into parallel light, an objective lens 6 for converging laser light on a magneto-optical recording medium 11, a phase compensation element 7 for adjusting a phase of light incident thereon, a Wollaston prism 8 for separating light incident thereon, a cylindrical lens 9 for generating astigmatism with respect to light incident thereon, and a photodetector 10 serving as a light-receiving element for converting light incident thereon into an electrical signal.
The semiconductor laser element 2, which is a light source for emitting light, emits laser light having a wavelength of 780 nm when the magneto-optical recording medium 11 is an MD or Hi-MD for example. The semiconductor laser element 2 is connected to an external circuit (not shown) for supplying a drive current, and the intensity of laser light can be changed by changing the amount of a current from the external circuit.
The grating 3 is a diffraction grating for separating the light emitted by the semiconductor laser element 2 into zero-order diffracted light, −first-order diffracted light and +first-order diffracted light. The beam splitter 4 transmits outgoing light emitted by the semiconductor laser element 2 toward the magneto-optical recording medium 11 and reflects return light reflected by the magneto-optical recording medium 11. The collimator lens 5 converts diffuse light emitted by the semiconductor laser element 2 into parallel light which then exits the lens.
For example, the objective lens 6 has a numerical aperture (NA) of 0.45, and is mounted on an actuator (not shown) for holding the objective lens 6 so as to be capable of being moved in a focus direction which is a direction in parallel with the optical axis of incident light and a tracking direction which is a direction in parallel with a radial direction of the magneto-optical recording medium 11. The objective lens 6 converges outgoing light which has been emitted by the semiconductor laser element 2, on an information recording surface of the magneto-optical recording medium 11 to form a light spot thereon.
The phase compensation element 7 imparts phase compensation to light incident thereon which is return light traveling from the magneto-optical recording medium 11 to reduce errors by suppressing crosstalk components from the lands and to thereby obtain favorable reproduction characteristics. Note that the phase compensation element 7 imparts phase compensation to light incident thereon in such an amount, which is the same for MD and Hi-MD, that satisfactory reproduction characteristics will be achieved in both of a case where the magneto-optical recording medium 10 is an MD and a case where the medium is a Hi-MD.
The Wollaston prism 8 separates the return light incident thereon which has been reflected by the magneto-optical recording medium 11 and beam splitter 4 and then transmitted by the phase compensation element 7, and the Wollaston prism 8 transmits the separated light by the cylindrical lens 9 to project the light in a predetermined light-receiving region on the after-described photodetector 10. The cylindrical lens 9 imparts astigmatism to light incident thereon so that the photodetector 10 can output a focus error signal (FE signal) The photodetector 10 is a signal conversion section having the predetermined light-receiving region thereon, which converts the laser light incident thereon into an electrical signal and performs calculations on the signal to output the above-mentioned FE signal, an RF signal, and a tracking error signal (TE signal).
The laser light emitted by the semiconductor laser element 2 is transmitted by the grating 3, beam splitter 4, and collimator lens 5 to enter the objective lens 6, and the light is converged on the information recording surface of the magneto-optical recording medium 11. The laser light converged on the information recording surface of the magneto-optical recording medium 11 is reflected on a reflecting surface of the magneto-optical recording medium 11, and transmitted by the objective lens 6 and collimator lens 5, and then reflected by the beam splitter 4, thereafter transmitted by the phase compensation element 7, and separated by the Wollaston prism 8, and further transmitted by the cylindrical lens 9 so as to be received by the photodetector 10. The photodetector 10 converts the received laser light into an electrical signal and outputs the signal.
In the optical head 1 disclosed in JP-A 2003-296960, the phase of light reflected by the magneto-optical recording medium 11 is properly adjusted by the phase compensation element 7, and the phase of light reflected by the lands is adjusted. Accordingly, in the optical head 1, crosstalk is suppressed and moreover, it is possible to prevent the degradation of reproduction characteristics of both of MDs or Hi-MDs serving as magneto-optical recording medium 11.
However, the optical head 1 disclosed in JP-A 2003-296960 has such a problem that insertion of the phase compensation element 7 increases cost and size of the optical head compared to an optical head having no phase compensation element. From this viewpoint, there has been a demand for an optical head having no such problems as increases in cost and size of the optical head, which can reduce errors by suppressing crosstalk components attributable to reflection on lands of a magneto-optical recording medium and thereby prevent degradation of reproduction characteristics.